1. Field of the Invention
The present invention relates to a data read/write system for a disc storage unit provided with servo information, such as a fixed disc storage unit, more particularly to a data read/write system for reading and writing data from and onto a sector in a track wherein a read/write head is displaced and positioned at a desired track while it is reading out servo information recorded on one or both surfaces of the disc, and a physical address is instructed in accordance with a logical address defined at the sector in the disc storage unit.
2. Description of the Prior Art
As is well known to those skilled in the art, there has always been a keen demand to increase storage capacity of a disc storage unit such as an external storage unit. In order to meet the demand, it is necessary that a number of tracks be defined on one or both surfaces of a hard disc. However, this leads to a reduction in the pitch between the tracks, which in turn causes errors in reading/writing even with a slight deviation in the position of the head. Recently, it has become a customary countermeasure to precisely position the head by closed loop control using servo information recorded on the disc or by a position encoder housed in an operating system for the disc.
In most cases, system for writing servo information is used called a servo-surface servo information system in which servo information is recorded on a specific surface of the disc. This system gives a counter result in that one of the surfaces of the disc, which are very precious, is occupied by servo information. However, subtle differences tend to occur between heads provided to respective disc surfaces due to such factors as mechanical errors and deviations in position of the heads from their normal position. Accordingly it is difficult to improve the accuracy of positioning of the heads beyond a certain degree by detecting deviation in position on a disc surface exclusively allotted to servo information and correcting the position of the head corresponding to another disc surface. The same is true for the system for positioning heads utilizing an encoder.
In order to solve the above-described problems, there has been used advantageously a so-called data-surface servo information system in which servo information is recorded on each disc surface for recording data. As for data-surface servo information systems there have been known a so-called concentrated servo information system in which servo information is recorded exclusively at one position in a peripheral or circumferential direction of a disc surface for recording data and a so-called built-in servo information system in which servo information is recorded dividedly so as to be distributed over a plurality of positions or between sectors.
Meanwhile, the area to which a head is to be positioned with reference to servo information and data reading or writing into is usually defined by a physical address which consist of a cylinder number, a head number and a sector number, which is a specific address system determined depending on how data are stored in a disc storage unit. However, since they must allow various combinations of different disc storage units with different physical address systems, computes usually give disc storage units instruction for defining an area for reading out and writing data from and into by a so-called logical address which is independent of the type of disc storage unit. The logical addresses consist of a number of sectors which are read/write units in the storage space of the disc storage unit and to which serial numbers are assigned. The disc storage unit initiates operations including head positioning and the like when it receives a logical or address range after converting it into a specific physical address range.
Referring to FIGS. 1A and 1B, which are each schematic views illustrating a disc surface for outlining addressing in a conventional data read/write system, the above-described intersector built-in servo information system and the conversion system in which logical addresses are converted into physical addresses as typical examples of the conventional read/write system will be briefly described.
On the two surfaces schematically shown in FIGS. 1A and 1B, there are shown a plurality of slender fan-shaped regions for servo information SI (eight in this example for convenience of illustration). Such servo information regions are arranged so as to wedge themselves in sectors g. Tens of sectors S are arranged in several hundreds of concentric tracks as shown in FIGS. 1A and 1B. Explanation of how to write servo information SI into the regions and the like will be made later hereinbelow with reference to FIG. 3.
Upon the positioning of a head above a specific track of one disc surface, the servo information SI written into the disc surface is read out via the head and an off-track amount is detected, defined as a deviation of the head from the center of the track, from the read-out signal. The head is controlled by closed loop control so that the off-track amount can be nulled. Therefore, there occurs no deviation in tracking based on mechanical errors between the heads. In this example, control of head position with reference to the servo information SI written into the respective intersector regions is continued during reading out and writing data from and into a sector S in a specific track on a specific disc surface.
In FIGS. 1A and 1B, the sectors S are assigned their own numbers which denote logical addresses instructed by the computer. The physical addresses on the side of the disc storage unit corresponding to the logical addresses consist of, for each disc surface, a cylinder number which is assigned, for example, from the outermost diameter track toward inner diameter tracks sequentially; a head number which is assigned corresponding to each disc surface; and a sector number which is assigned to a sector in each track on each disc surface. Each of the numbers starts from 0 for the sake of convenience. Usually, in the conventional system, the logical addresses instructed by the computer are converted into physical addresses in the disc storage unit as follows.
As shown in FIGS. 1A and 1B, the leading logical addresses 0 to 7 are converted sequentially to eight sectors in the radially outermost No. 0 track on a second disc surface with the head number 0, and subsequently as shown in FIG. 1B, the logical addresses 8 to 15 are converted into eight sectors in the radially outermost No. 0 track on a second disc surface with the head number 1.
When number of the disc surfaces is two, as in the above-described example, subsequent logical addresses 16 to 23 are converted into sectors in the track with the cylinder number 1 which is one track inward with respect to the radially outermost track on the disc surface with the head number 0 as shown in FIG. 1A, and further logical addresses 24 to 31 are convented into sectors in the track with the cylinder number 1 on the disc surface with the head number 1 shown in FIG. 1(B). Thereafter, similar conversion is performed sequentially. Assuming that the total number of sectors in a single disc surface is indicated by x, the last logical address is 2x-1 and is converted into address in the last sector in the radially innermost track on the disc surface with the head number 1 shown in FIG. 1B.
The conventional address conversion described above has an advantage that upon converting the contents of data information to be read out or written into, for example, from the logical addresses 0 to 7 to the ones 8 to 15, the operation can be continued by merely switching on or off the heads without changing the position thereof. That is, since the tracks on the same radial position in a plurality of disc surfaces form a cylinder, they are assigned the same cylinder number, and in order to make use of the above-described advantage, the logical addresses are sequentially converted into physical addresses in the tracks with the same cylinder number.
According to the above-described conventional read/write system, it is expected that of off-tracking of head a during its head/write action could be reduced by employing a built-in servo information system in which servo information is recorded on each disc surface. Also the amount of displacement of the head could be made as small as possible to reduce means access time of the device by using a kind of disc surface parallel addressing system in which a series of logical addresses are continuously converted into physical addresses with the same cylinder number as much as possible. However, it is necessary that mechanical precision of a head operating system be improved in its own way before such effect can be actually obtained, and a problem arises that when reducing the pitch between any adjacent tracks in order to increase storage capacity is contemplated it becomes increasingly difficult to increase the mechanical precision of the device accordingly.
Of factors affecting the mechanical precision of the device, the most important one is fluctuation in pitch precision of a head supported by a thin plate spring which would readily occur from head to head. It follows, therefore, that tracking errors occur upon the switching of the head and its takes a long time to correct such errors, resulting in that the effect of reducing the access time cannot be obtained virtually.
That is, in the above-described disc surface parallel addressing system, when data with a series of logical addresses is read or written, reading or writing the data from or into tracks on a different disc surface with the same cylinder number can be accomplished by merely switching heads. Therefore, it is natural to expect that the read/write operation can essentially be performed continuously. The above-described pitching problem may be considered as a mechanical fall down or inclination which would affect the radial position of the head over the disc surface, and fluctuation, if any, from head to head results in, when servo information is read out immediately after switching the head, excessive tracking error being detected therefrom and a reading/writing operation will no longer be continued as it is. Of course, the tracking error can be corrected immediately. However, the disc must be turned at least one before reading/writing can be initiated again, which prolongs the access time accordingly.
In practice, yawing of the head or the like occurs which causes a radial deviation in addition to the pitching problem, with the result that mechanical error increases and the advantage tends to be thereby off-set.